Integrated optical circuits are currently manufactured from a wafer of a semiconductor material, or semiconductor wafer, which is then sawn to obtain a plurality of individual chips. Before the sawing step, tests are performed to verify that the optical circuits are functional.
To test an integrated optical circuit, optical signals are supplied to optical inputs of the circuit, and the values of the corresponding output signals are observed, which output signals may be optical signals available at the optical outputs of the circuit. The optical signals are simultaneously supplied to a plurality of optical inputs of the circuit by means of a block of optical fibers, that is, a parallelepipedal block having a plurality of parallel, coplanar, and regularly distributed optical fibers held in place therein. The ends of these fibers are flush with the lower surface of the block and are distributed as optical inputs and/or outputs of the circuit. Thus, by properly positioning the fiber block above the optical inputs and/or outputs, each optical fiber has its end arranged above a corresponding optical input or output.
For the test to be correct, the distances between the ends of the fibers and the corresponding optical inputs or outputs should be identical, for example, to within ±2.5 μm, for all the block fibers. The ends of the fibers all being vertically in line with the lower surface of the block, this surface should be as parallel as possible to the upper surface of the chip or of the semiconductor wafer comprising the optical circuit to be tested. In practice, the semiconductor chip is arranged on the upper surface of an XY table and it is sufficient for the lower surface of the block to be parallel to the upper surface of the XY table.
A known method to adjust the parallelism of these two surfaces is based on the use of a plurality of cameras arranged around the fiber block to view the orientation of its lower surface with respect to the upper surface of the semiconductor chip or of the XY table. The orientation of the block is then adjusted until the surfaces are made substantially parallel. It is however difficult to find a compromise between the enlargement and the field of each camera which enables to simultaneously view the fiber block and the upper surface of the chip or of the table while keeping a sufficient accuracy. It is also difficult to find a compromise between the cost of cameras and their resolution, which conditions the accuracy of the adjustment. This method thus has a low accuracy and a high cost. Further, its implementation may turn out being complex, all the more as transparent materials are difficult to accurately observe with a camera: there are refraction and reflection problems which limit the resolution of the camera and which make the contours fuzzy.